Tathata has been described as the ultimate goal of Buddhism. What is tathata? What is the closest thing we experience in everyday life close to tathata? How can we attain knowledge of tathata?
For starters, here is the Theravada perspective, with some quotes:
THUSNESS (by ven. Buddhadasa Bhikkhu)
Now, we come to the fourth and last topic: tathata (suchness, thusness). "Merely thus," "just such": everything is such as it is and in no way different from that thusness. This is called "tathata." When tathata is seen, the three characteristics of anicca, dukkha, and anatta are seen, sunnata is seen, and idappaccayata is seen. Tathata is the summary of them all -- merely thus, only thus, not-otherness. There is nothing better than this, more than this, other than this, thusness. To intuitively realize tathata is to see the truth of all things, to see the reality of the things which have deceived us. The things which delude us are all the things which cause discrimination and duality to arise in us: good-evil, happiness-sadness, win-lose, love-hate, etc. There are many pairs of opposites in this world. By not seeing tathata, we allow these things to trick us into believing in duality: this-that, liking-disliking, hot-cold, male-female, defiled, enlightened. This delusion causes all our problems. Trapped in these oppositions, we can't see the truth of things. We fall into liking and disliking, which in turn leads to the defilements, because we don't see tathata.
From SN 12.20:
Iti kho, bhikkhave, yā tatra tathatā avitathatā anaññathatā idappaccayatā – ayaṃ vuccati, bhikkhave, paṭiccasamuppādo.
What's there in this way is a reality, not an unreality, not other than what it seems, conditioned by this/that. This is called dependent co-arising. (Thanissaro)
Thus bhikkhus, the actuality in this, the inerrancy, the not-otherwiseness, specific conditionality: this is called dependent origination. (Bhikkhu Bodhi)
From AN 4.24:
A Tathagata is a seer of what is to be seen, but he does not mind the seen, the unseen, the seeable, the seer. So likewise with the heard, the sensed and the cognized: he thinks of none of these modes of theirs. Therefore among things seen, heard, sensed and cognized he is precisely 'such'. Moreover, than he who is 'such' there is no other 'such' further or more excellent.
So, tathata is the culmination of the Third Noble Truth, cessation of suffering. When you no longer crave any experiences, don't avert any experiences, don't project any delusions onto your experience - when there is no longer a mismatch between "this" and imaginary "that" - that's tathata.